This invention relates to improved circuitry for color television receivers and more particularly to a picture level control circuit including compatible automatic chroma control for such receivers.
Color television receivers of the prior art are generally equipped with manual controls allowing individual adjustment of luminance (contrast), chrominance (color saturation), tint, brightness and various other functions of the receivers. These controls due to their number and interrelationship, have tended to make color television receiver adjustment both complex and time consuming. It is desirable to eliminate such complex adjustment of color television receivers and accordingly single knob picture controls have been designed. Such controls may ideally be preadjusted in the factory to set the ratio between chrominance and luminance. However, since factory settings do not always correspond to viewer preference controls are set to provide the desired ratio between chrominance and luminance and then the single picture control may be used to simultaneously adjust luminance and chrominance.
The picture control can be placed in a color television either prior to the separation of the video signal into chrominance and luminance components or after such separation. It is desirable to place the picture control prior to the separation of the video since here no problem of chroma tracking luminance is encountered. However, such placement exposes the adjustment of the video level to automatic chroma level compensating circuits in succeeding stages. Such compensating circuits comprise the color burst automatic gain control circuit and the chroma averaging automatic gain control circuit, which circuits would ordinarily treat any changes in chroma level brought about by the picture control or any other source as unwanted changes in chroma level and compensate to keep the chroma level constant.
If the picture level control is placed in the receiver subsequent to the separation of the video signal into its luminance and chrominance components, it is possible to avoid the reach of these automatic compensating circuits and their chroma level change defeating action. However, such an approach requires incorporating much additional circuitry. Accordingly, in order to utilize existing circuitry, the present invention contemplates placement of the picture control circuit either prior to the color burst AGC circuit where composite video signal level is controlled or prior to the chroma averaging AGC circuit where separate luminance and chrominance signals are simultaneously controlled. In both configurations, the picture control circuit works in conjunction with existing circuitry including the AGC circuits to allow picture level adjustment.